Concept is a central entity, some kind of a hub, which binds and defines the roles of all other entities. Every entity present in the model, should be defined as a Concept.

Requirements:
  * must have name
  * must belong to [[documentation:athena:entity_domain|Domain]]
  * must be declared in [[documentation:athena:entity_vocabulary|Vocabulary]]
  * must be classified by [[documentation:athena:entity_class|Concept Class]]
  * must have a code from source vocabulary
  * must have a defined validity period

Options:
  * might be Classification, Standard or non-standard Source Concept(default)
  * might be invalidated. "D" - deleted, "U" - updated, NULL - concept is valid(default)
Each Concept has its own unique ID.